For whatever it is worth, I doubt Apple will come out with a larger screen anytime soon.
First off, they are obligated (to some extent) to maintain backwards compatibility with current apps. That's why the ipad mini used the same resolution as the ipad2 - so developers didn't need to recode their apps (That and the 163PPI screen is the same resolution as the 3GS *hint*), and why the iphone5 has the same width as the iphone4 (so unscaled apps don't look too crap).
A larger screen would degrade the screen's PPI to well below their own threshold for what constitutes retina (a mockup showed that a 4.9 inch model could be had at roughly 264PPI, same as ipad4). This would cause them to lose one of the key selling points of their iphone. I suppose Apple could, in theory at least, quadruple the number of pixels (double the number of pixels on each side), but that would probably tax the GPU immensely, not to mention the absurd drain on battery life, and the pointlessness of going beyond 1920x1080 resolution.
I suppose in this context, their retina display is their biggest strength, and their greatest weakness, because of the limitations it places when you want to change your screen size.